Concrete is the most widely used construction material in the world — a mixture of cement, fine aggregate (sand), coarse aggregate (stone or gravel) and water that begins as a workable paste and cures into a hard, stone-like solid. The cement and water react chemically (hydration) to bind the aggregates together; the aggregates give the mass its bulk, strength and economy.
The proportions of these ingredients determine the concrete's grade and strength. A leaner mix (more aggregate) is cheaper but weaker; a richer mix (more cement) is stronger but costlier and more prone to shrinkage. In India, concrete is specified by grade — M15, M20, M25 and so on — where the number is the characteristic compressive strength in N/mm² at 28 days.
Because dry materials occupy more space than the finished wet concrete, quantities are estimated by multiplying the wet volume by a bulking factor of about 1.54 before splitting by the mix ratio.